The present invention is directed to printheads useful for thermal ink jet printing processes. More specifically, the present invention is directed to thermal ink jet printheads having advantages such as improved ink resistance and channel and nozzle features with improved aspect ratio. One embodiment of the present invention is directed to a thermal ink jet printhead which comprises: (i) an upper substrate, and (ii) a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes formed thereon, said lower substrate having an insulative layer deposited on the surface thereof and over the heating elements and addressing electrodes and patterned to form recesses therethrough to expose the heating elements and terminal ends of the addressing electrodes, said upper and lower substrates being bonded together to form a thermal ink jet printhead having droplet emitting nobles defined by the upper substrate, the insulative layer on the lower substrate, and the heating elements in the lower substrate, wherein at least one of said upper substrate and said insulative layer comprises a crosslinked polymer formed by crosslinking a precursor polymer which is a phenolic novolac resin having glycidyl ether functional groups on the monomer repeat units thereof. Another embodiment of the present invention is directed to a process for forming a thermal ink jet printhead which comprises: (a) providing a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes having terminal ends formed thereon; (b) depositing onto the surface of the lower substrate having the heating elements and addressing electrodes thereon a layer comprising a precursor polymer which is a phenolic novolac resin having glycidyl ether functional groups on the monomer repeat units thereof, (c) exposing the layer to actinic radiation in an imagewise pattern such that the precursor polymer in exposed areas becomes a crosslinked polymer and the precursor polymer in unexposed areas does not become crosslinked, wherein the unexposed areas correspond to areas of the lower substrate having thereon the heating elements and the terminal ends of the addressing electrodes; (d) removing the precursor polymer from the unexposed areas, thereby forming recesses in the layer, said recesses exposing the heating elements and the terminal ends of the addressing electrodes; (e) providing an upper substrate; and (f) bonding the upper substrate to the lower substrate to form a thermal ink jet printhead having droplet emitting nozzles defined by the upper substrate, the crosslinked polymer on the lower substrate, and the heating elements in the lower substrate.
In microelectronics applications, there is a great need for low dielectric constant, high glass transition temperature, thermally stable, photopatternable polymers for use as interlayer dielectric layers and as passivation layers which protect microelectronic circuitry. Poly(imides) are widely used to satisfy these needs; these materials, however, have disadvantageous characteristics such as relatively high water sorption and hydrolytic instability. There is thus a need for high performance polymers which can be effectively photopatterned and developed at high resolution.
One particular application for such materials is the fabrication of ink jet printheads. Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system""s ability to produce high quality copies.
Another type of drop-on-demand system is known as thermal ink jet or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the xe2x80x9cbubble jetxe2x80x9d system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280xc2x0 C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
The present invention is suitable for thermal ink jet printing processes.
In ink jet printing, a printhead is usually provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium such as paper by expelling droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus.
In thermal ink jet printing, a thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. The rapidly expanding vapor bubble pushes the column of ink filling the channel towards the nozzle. At the end of the current pulse the heater rapidly cools and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle as an ink drop. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
Ink jet printheads include an array of nozzles and may, for example, be formed of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on silicon wafers in a highly precise manner. Moreover, these structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by using the above-mentioned ODE techniques typically comprise a cover or channel plate which contains a plurality of nozzle-defining channels located on a lower surface thereof bonded to a heater plate having a plurality of resistive heater elements formed on an upper surface thereof and arranged so that a heater element is located in each channel. The upper surface of the heater plate typically includes an insulative layer which is patterned to form recesses exposing the individual heating elements. This insulative layer is referred to as a xe2x80x9cpit layerxe2x80x9d and is sandwiched between the cover or channel plate and heater plate. For examples of printheads employing this construction, see U.S. Pat. Nos. 4,774,530 and 4,829,324, the disclosures of each of which are totally incorporated herein by reference. Additional examples of thermal ink jet printheads are disclosed in, for example, U.S. Pat. Nos. 4,835,553, 5,057,853, and 4,678,529, the disclosures of each of which are totally incorporated herein by reference. Alternatively, the cover plate can be flat, without any nozzle-defining channels therein, and the channel or nozzle walls can be defined by the recesses in the insulative layer.
U.S. Pat. No. 5,762,812 (Narang), the disclosure of which is totally incorporated herein by reference, discloses a thermal ink jet printhead which comprises (a) an upper substrate with a set of parallel grooves for subsequent use as ink channels and a recess for subsequent use as a manifold, the grooves being open at one end for serving as droplet emitting nozzles; and (b) a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes formed thereon, the lower substrate having a thick film insulative layer deposited over the heating elements and addressing electrodes and patterned to form recesses therethrough to expose the heating elements and terminal ends of the addressing electrodes; said upper and lower substrates being aligned, mated, and bonded together to form the printhead with the grooves in the upper substrate being aligned with the heating elements in the lower substrate to form droplet emitting nozzles, wherein the upper and lower substrates are bonded together with an adhesive which comprises a reaction product of (a) an epoxy resin selected from the group consisting of (1) those of the formula 
wherein n is an integer of from 1 to about 25; (2) those of the formula 
wherein n is an integer of from 1 to about 25; (3) those of the formula 
and (4) mixtures thereof; and (b) a curing agent which enables substantial curing of the epoxy resin at a temperature of not lower than the softening point of the resin and not higher than about 20xc2x0 C. above the softening point of the resin within a period of no more than about 3 hours. Also disclosed are processes for preparing a thermal ink jet printhead with the aforementioned adhesive components.
U.S. Pat. No. 5,945,253 (Narang et al.), the disclosure of which is totally incorporated herein by reference, discloses a composition which comprises a polymer containing at least some monomer repeat units with photosensitivity-imparting substituents which enable crosslinking or chain extension of the polymer upon exposure to actinic radiation, said polymer being of the formula 
wherein x is an integer of 0 or 1, A is one of several specified groups, such as 
B is one of several specified groups, such as 
or mixtures thereof, and n is an integer representing the number of repeating monomer units, wherein said photosensitivity-imparting substituents are allyl ether groups, epoxy groups, or mixtures thereof. Also disclosed are a process for preparing a thermal ink jet printhead containing the aforementioned polymers and processes for preparing the aforementioned polymers.
U.S. Pat. No. 4,882,245 (Gelorme et al.), the disclosure of which is totally incorporated herein by reference, discloses a photocurable composition which is useful as a permanent resist in the manufacture of printed circuit boards and packages of such boards comprises a multifunctional epoxidized resin, a reactive diluent, a cationic photoinitiator, and, optionally, an exposure indicator, a coating aid and a photosensitizer.
U.S. Pat. No. 5,026,624 (Day et al.), U.S. Pat. No. 5,278,010 (Day et al.), and U.S. Pat. No. 5,304,457 (Day et al.), the disclosures of each of which are totally incorporated herein by reference, disclose an improved photoimagable cationically polymerizable epoxy based coating material. The material includes an epoxy resin system consisting essentially of between about 10 percent and about 80 percent by weight of a polyol resin which is a condensation product of epichlorohydrin and bisphenol A having a molecular weight of between about 40,000 and 130,000; between about 20 percent and about 90 percent by weight of an epoxidized octafunctional bisphenol A formaldehyde novolak resin having a molecular weight of 4,000 to 10,000; and if flame retardancy is required between about 35 percent and 50 percent by weight of an epoxidized glycidyl ether of tetrabromo bisphenol A having a softening point of between about 60xc2x0 C. and about 110xc2x0 C. and a molecular weight of between about 600 and 2,500. To this resin system is added about 0.1 to about 15 parts by weight per 100 parts of resin of a cationic photoinitiator capable of initiating polymerization of said epoxidized resin system upon exposure to actinic radiation; the system being further characterized by having an absorbance of light in the 330 to 700 nanometer region of less than 0.1 for a 2.0 mil thick film. Optionally a photosensitizer such as perylene and its derivatives or anthracene and its derivatives may be added.
U.S. Pat. No. 5,859,655 (Gelorme et al.), the disclosure of which is totally incorporated herein by reference, discloses an ink jet printer head formed from a photoimageable organic material. This material provides for a spin-on epoxy based photoresist with image resolution and adhesion to hard to bond to metals such as gold or tantalum/gold surfaces that are commonly found in such printer applications. When cured, the material provides a permanent photoimageably defined pattern in thick films ( greater than 30) that has chemical (i.e. high pH inks) and thermal resistance.
U.S. Pat. No. 5,907,333 (Patil et al.), the disclosure of which is totally incorporated herein by reference, discloses an ink jet printhead having ink passageways formed in a radiation cured resin layer which is attached to a substrate. The passageways are connected in fluid flow communication to an ink discharging outlet provided by an orifice plate. To form the passageways in the resin layer, a resin composition is exposed to a radiation source in a predetermined pattern to cure certain regions of resin layer while other regions which provide the passageways remain uncured. The uncured regions are removed from the resin layer leaving the desired passageways. The resin composition to be used for forming the radiation curable layers is a resin composition comprising a first multifunctional epoxy compound, a second multifunctional compound, a photoinitiator, and a non-photoreactive solvent.
WO98/07069 (Mastrangelo et al.), the disclosure of which is totally incorporated herein by reference, discloses polymer-based microelectomechanical system (MEMS) technology suitable for the fabrication of integrated microfluidic systems, particularly medical and chemical diagnostics system, ink jet printer head, as well as any devices that require liquid- or gas-filled cavities for operation. The integrated microfluidic systems may consist of pumps, valves, channels, reservoirs, cavities, reaction chambers, mixers, heaters, fluidic interconnects, diffusers, nozzles, and other microfluidic components on top of a regular circuit substrate. The technology is superior to alternatives such as glass-based, polysilicon-based MEMS technology as well as hybrid xe2x80x9ccircuit boardxe2x80x9d technology because of its simple construction, low cost, low temperature processing, and ability to integrate any electronic circuitry easily along with the fluidic parts.
U.S. Pat. No. 6,124,372 (Smith et al.), the disclosure of which is totally incorporated herein by reference, discloses a composition comprising a polymer with a weight average molecular weight of from about 1,000 to about 100,000, said polymer containing at least some monomer repeat units with a first, photosensitivity-imparting substituent which enables crosslinking or chain extension of the polymer upon exposure to actinic radiation, said polymer also containing a second, thermal sensitivity-imparting substituent which enables further crosslinking or chain extension of the polymer upon exposure to temperatures of about 140xc2x0 C. and higher, wherein the first substituent is not the same as the second substituent, said polymer being selected from the group consisting of polysulfones, polyphenylenes, polyether sulfones, polyimides, polyamide imides, polyarylene ethers, polyphenylene sulfides, polyarylene ether ketones, phenoxy resins, polycarbonates, polyether imides, polyquinoxalines, polyquinolines, polybenzimidazoles, polybenzoxazoles, polybenzothiazoles, polyoxadiazoles, copolymers thereof, and mixtures thereof.
U.S. Pat. No. 6,139,920 (Smith et al.), the disclosure of which is totally incorporated herein by reference, discloses a composition comprising a blend of (a) a thermally reactive polymer selected from the group consisting of resoles, novolacs, thermally reactive polyarylene ethers, and mixtures thereof; and (b) a photoreactive epoxy resin that is photoreactive in the absence of a photocationic initiator. Also disclosed is a thermal ink jet printhead prepared with the composition.
In the fabrication of sideshooter-type printhead elements, the fluidic pathway is often defined by a photopatternable polyimide negative photoresist. Polyimides provide thermally stable structures and possess good adhesion. Polyimides, however, are not ideal because of their frequent hydrolytic instability in alkaline aqueous media and because of the high shrinkage (sometimes up to about 40 percent) observed for features during final cure caused by the imidization process. Accordingly, there is a need for chemically stable, hydrolytically stable, and solvent resistant negative resists for sideshooter ink jet printheads. As the sideshooter ink jet printhead has evolved, a need has also arisen for resist materials that can be patterned at high aspect ratio and that do not suffer from loss of resolution through shrinkage.
While known compositions and processes are suitable for their intended purposes, a need remains for improved sideshooter thermal ink jet printheads. In addition, a need remains for sideshooter thermal ink jet printheads that contain chemically stable materials. Further, a need remains for sideshooter thermal ink jet printheads that are hydrolytically stable in aqueous media, particularly alkaline aqueous media. Additionally, a need remains for sideshooter thermal ink jet printheads that are formed of photopatternable materials that exhibit low shrinkage upon curing. There is also a need for sideshooter thermal ink jet printheads that are solvent resistant. In addition, there is a need for sideshooter thermal ink jet printheads that can be patterned at high aspect ratio and that do not suffer from loss of resolution through shrinkage. Further, there is a need for sideshooter thermal ink jet printheads that are formed of photopatternable materials that exhibit low swelling when subjected to solvent development subsequent to photoexposure and also exhibit low swelling upon exposure to solvents and aqueous media commonly used in ink jet inks. Additionally, there is a need for sideshooter thermal ink jet printheads that are formed of photopatternable materials of good lithographic sensitivity. A need also remains for sideshooter thermal ink jet printheads that are formed of thermally stable materials. In addition, a need remains for sideshooter thermal ink jet printheads that are formed of photopatternable polymers that, when applied to printhead elements by spin casting techniques and cured, exhibit reduced edge bead and no apparent lips and dips. Further, a need remains for sideshooter thermal ink jet printheads that are formed of photopatternable polymers that can be exposed without the need for mask biasing. Additionally, a need remains for thermal ink jet printheads of sideshooter configuration that enable high nozzle density, including densities of 1,200 dots per inch or more. There is also a need for sideshooter thermal ink jet printheads that are formed of photopatternable polymers that exhibit clean, sharp, square edges of the patterned features. In addition, there is a need for sideshooter thermal ink jet printheads that are formed of photopatternable materials that enable reduced or no need for polishing subsequent to patterning. Further, there is a need for sideshooter thermal ink jet printheads that are formed of photopatternable materials wherein the mask through which the photopatternable materials are exposed can be reproduced while retaining uniform film thickness across the wafer and features. Additionally, there is a need for sideshooter thermal ink jet printheads that are formed of photopatternable materials that enable a wide variety of drop volumes. A need also remains for sideshooter thermal ink jet printheads that are formed of photopatternable materials that enable a variety of cleanly defined nozzles of different dimensions and that produce different drop volumes in the same printhead.
The present invention is directed to a thermal ink jet printhead which comprises: (i) an upper substrate, and (ii) a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes formed thereon, said lower substrate having an insulative layer deposited on the surface thereof and over the heating elements and addressing electrodes and patterned to form recesses therethrough to expose the heating elements and terminal ends of the addressing electrodes, said upper and lower substrates being bonded together to form a thermal ink jet printhead having droplet emitting nozzles defined by the upper substrate, the insulative layer on the lower substrate, and the heating elements in the lower substrate, wherein at least one of said upper substrate and said insulative layer comprises a crosslinked polymer formed by crosslinking a precursor polymer which is a phenolic novolac resin having glycidyl ether functional groups on the monomer repeat units thereof. Another embodiment of the present invention is directed to a process for forming a thermal ink jet printhead which comprises: (a) providing a lower substrate in which one surface thereof has an array of heating elements and addressing electrodes having terminal ends formed thereon; (b) depositing onto the surface of the lower substrate having the heating elements and addressing electrodes thereon a layer comprising a precursor polymer which is a phenolic novolac resin having glycidyl ether functional groups on the monomer repeat units thereof; (c) exposing the layer to actinic radiation in an imagewise pattern such that the precursor polymer in exposed areas becomes a crosslinked polymer and the precursor polymer in unexposed areas does not become crosslinked, wherein the unexposed areas correspond to areas of the lower substrate having thereon the heating elements and the terminal ends of the addressing electrodes; (d) removing the precursor polymer from the unexposed areas, thereby forming recesses in the layer, said recesses exposing the heating elements and the terminal ends of the addressing electrodes; (e) providing an upper substrate; and (f) bonding the upper substrate to the lower substrate to form a thermal ink jet printhead having droplet emitting nozzles defined by the upper substrate, the crosslinked polymer on the lower substrate, and the heating elements in the lower substrate.